Coating of sheet material



Dzc. 16, 1969 N. o. CLARK L 3,484,279

COATING 0F SHEET MATERIAL Filed April 26, 1966 2 Sheets-Sheet 1 Dec. 16, 1969 N, o. CLARK ET AL 3, 34,279

. I comma 0F SHEET MATERIAL Filed April 26, 1966 2 Sheets-Sheet 2 United States Patent 0 U.S. Cl. 117-111 7 Claims ABSTRACT OF THE DISCLOSURE A method and apparatus for coating moving sheet material is disclosed wherein the substrate is contacted by a predetermined constant level of coating liquid as it moves over a support roll. The apparatus comprises a scraper blade and a partition disposed within the area formed by the said scraper blade and the support roll thereby forming a receiving area and an application area with a restricted orifice being defined therebetween. Level sensing means disposed in the application area to maintain a constant level of coating material is operatively connected to pump means for regulating the amount of material delivered to the receiving area.

This invention relates to the coating of sheet material and is more especially but not exclusively concerned with the application of a coating colour mixture to a continuous sheet material such as paper.

By the term coating colour mixture there is meant a mixture of mineral pigment and adhesive. Kaolin is the most widely used pigment and aqueous solutions of starch or casein are widely used as adhesives. Synthetic latices are also used as adhesives.

The known method of applying a coating colour mixture to the surface of a continuous sheet of paper using the mechanism known as the trailing blade, for example as described in United States patent specification No. 2,368,176 is particularly favoured because of its economy, simplicity and efficiency. Furthermore, the trailing blade process operates best at high speed in the region of LOGO-3,000 ft./min. and applies a coating which results in a particularly flat surfacewhich is desirable for printing purposes.

In the two basic, well established forms of this process,

there is employed a rubber covered roller over which the paper is led, the roll having a diameter of the order of 3 feet and being suflicienty wide for the width of paper being coated. In both arrangements the blade is held in contact with the paper at a position where the paper is supported by the roll, the blade being at an angle to the tangent of the roll at that position i.e. at an acute angle to the paper surface and in the trailing position with respect to the direction of movement of the paper surface.

In the first general form, known as the trough system, for example as described in United States patent specification No. 3,169,082 the trailing blade is held against a descending surface of the paper and with the help of end dams forms a trough in which a coating colour mixture is held to a depth of a few inches. In the second form, known as the inverted blade system, the trailing blade is held against the ascending surface of the paper and removes excess coating colour mixture which has been applied to the paper a few inches in advance of the blade by a pick-up roller dipping into a trough of coating colour mixture. Such an inverted blade system is described in United States patent specification No. 3,150,002. In both cases the edge of the trailing blade in contact with the paper touches the paper at a point where it is sup ported by the rubber coated roller.

In both of these arrangements the coating colour mixture is in contact with the paper for a period of time related to the linear speed of the paper through the machine and the distance of wet contact which, in the trough system, may be of the order of 6-8 inches and in the inverted blade system of the order of 8-12 inches. Thus in both these known forms of the process there is time and opportunity for a proportion of the adhesive solution to filter away from the coating colour mixture into the absorbent paper before the passage of the coated paper under the edge of the trailing blade defines the thickness of the coating.

- The achievement of satisfactory coating depends on the rheology of the coating colour mixture on the paper at'the time when the paper passes under the edge of the blade whereby the thickness of the coating is defined. The rheology is determined largely by the concentration of adhesive and mineral in the coating colour mixture and by the properties of these materials exhibited at the concentrations used.

It is generally desirable to run the coating apparatus at maximum possible speed to achieve maximum output and the speed of the apparatus is frequently limited by the evaporative capacity of the subsequent drying section. The coating has to be dried on the sheet of paper before the paper can be rolled up for removal and this is achieved generally in a continuous process in which the wet coated paper is passed over steel drying cylinders or under high velocity hot air drying hoods. If, therefore, the loss of water in the form of adhesive solution into the base paper is great and a certain defined proportion of water is necessary in the coating colour mixture on the paper in order to achieve the correct rheology at the time of contact of the coated paper with the trailing blade, then a larger quantity of water must be present in the original coating colour mixture and hence more water needs to be evaporated during the subsequent drying process. This may severely limit the speed at which the coating apparatus can be operated. If however the loss of water into the base paper is small then less Water has to be evaporated in the drying section and hence the coating apparatus can be run faster with greater output and economy.

Attempts have hitherto been made to meter the quantity of coating colour mixture applied in an inverted blade machine by projecting the desired quantity of coating colour mixture, or a slight excess thereof, by spray or jets on to the paper surface in front of the trailing blade so that the excess is removed by the latter.

According to one aspect of the present invention there is provided a method of applying a coating composition to a sheet material by the trailing blade. technique, which comprises moving sheet material with respect to and in contact with an edge of a blade disposed at an acute angle to the sheet material and in the trailing position with respect to the direction of movement of the sheet material, providing a partition disposed in the acute angle defined by the blade and the sheet material and spaced apart from the surface of the sheet material, said partition defining a gap or restricted orifice between said edge of the blade and an edge of the partition, and introducing coating composition between the blade and the partition whereby a metered quantity of the coating composition is applied to a limited area of the sheet material through the said gap.

According to another aspect of the present invention, there is provided an apparatus for applying a coating composition to a sheet material comprising a means for supporting a moving sheet material, a blade at an acute angle to the sheet material in use and in the trailing position with respect to the intended direction of movement' of the sheet material, said blade having a forward edge adapted to contact the sheet material in use at a position where it is supported by said means, and a partition disposed in use of the apparatus in the acute angle between the sheet material and the blade and spaced from the sheet material, said partition having a forward edge disposed adjacent to said blade so that a restricted orifice is defined therebetween through which a metered amount of the coating composition may be introduced to and deposited on a limited area of said sheet material.

By means of the present invention it is possible to prevent contact of the coating composition and the sheet material up to a point very close to the contacting edge of the trailing blade.

Advantageously the coating composition is a colour mixture comprising a mixture of mineral and adhesive solution and the sheet material is paper. The means for supporting the moving sheet material is preferably a roller covered with a resilient material such as rubber.

The trailing blade may be applied to a descending surface of the sheet material and the invention can be carried out by the incorporation of a partition in the form of a diaphragm which extends down from the top of a trough formed between the sheet material and the blade to a point very close to the coating edge of the trailing blade said diaphragm being in close proximity to but spaced from the sheet material. The mass of coating composition is disposed between the trailing blade and the diaphragm the lower edge of the diaphragm and the coating edge of the trailing blade defining a gap or restricted orifice through which coating composition can be applied to the sheet material. In this way the period of wet contact of the coating composition with the sheet material is very much reduced, since the bulk of the mass of the coating composition is prevented from contacting the sheet material by the diaphragm. When the coating composition is in the form of, for example, a coating colour mixture, and the sheet material is absorbent, for example paper, the composition can be applied in a more concentrated form without leading to an undesirable high viscosity at the trailing blade coating edge due to loss of water into the paper.

The coating composition can be applied through a restricted orifice formed between the trailing blade used for the coating application and a partition in the form of a second flexible or non-flexible blade held above the coating blade in such a position that it forms an appropriate gap for egress of the coating composition onto the sheet material at a point close to the position where the coated sheet material passes under the trailing blade.

If desired, the coating composition introduced through the gap between the trailing blade edge and the edge of the partition may be allowed to collect to a certain extent in the nip formed between the sheet material on the one hand and the trailing blade applying edge and the partition on the other hand. In this case, the coating composition may be introduced between the trailing blade and the partition by means of a pump, the quantity of coating material metered by the pump being controlled by the level of coating composition maintained in the nip.

In accordance with the present invention the trailing blade may be used in the inverted position in conjunction with an ascending sheet material. An excess of coating composition can then be pumped through the gap between the trailing blade edge in contact with the sheet material and the partition edge spaced from the sheet material and into contact with the surface of the sheet material. The coating composition in excess of that which can be applied to the sheet material is then allowed to run down over the top of the gap-defining partition edge and may be collected and re-introduced between the blade and the partition.

By means of the present invention, when the coating composition is, for example, a coating colour mixture and the sheet material is absorbent, for example paper, the sheet material can be protected from early wet contact with the coating composition by the partition and thus the migration of adhesive solution from the colour mixture into the paper may be controlled. Hence the formulation of the coating colour mixture is facilitated and the quantity of water to be evaporated for a given weight of coating applied is decreased. The process of the present invention can be carried out in a clean manner and the occurrence of impurities in the coating can be avoided.

For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by Way of example, to the accompanying drawings, in which:

FIGURE 1 shows in diagrammatical form a first embodiment of the invention in which the sheet material is moved in a downward direction,

FIGURE 2. shows in diagrammatical form a second embodiment of the invention in which the pump and level sensing apparatus are shown, and

FIGURE 3 shows in diagrammatical form a third embodiment of the invention in which the sheet material is moved in an upward direction.

Referring to FIGURE 1, there is shown a sheet material 1 and a supporting means therefor comprising a rubber covered roller 2. The direction of movement of the sheet material when the apparatus is in use is indicated by the arrows. In this embodiment the sheet material 1 is moved in a downward direction whilst being coated. A blade 3, which is in operative association with roller 2, is provided at an acute angle to the sheet material 1 and in a trailing position with respect to the direction of movement of the sheet material 1 and has an edge contacting the surface of the sheet material at a position where the sheet material is supported. A partition 4 in the form of a diaphragm is also provided a forward edge of which, together with the surface-contacting or forward edge of blade 3, defines gap or restricted orifice 5. Partition 4 is spaced from the sheet material 1.

In use a coating colour mixture or coating composition 6 is introduced between the blade 3 and the diaphragm 4 and through gap 5 whereby the coating colour mixture becomes applied to the sheet material 1, after which it is scraped by blade 3 in order to obtain the desired thickness. The sheet material is thus only in wet contact with the coating colour mixture for a minimum length of time.

Referring to FIGURE 2, there is shown a second embodiment of the invention, like parts of this embodiment being designated by the same reference numerals as those used in FIGURE 1. The coating composition 6 is introduced between the blade 3 and the partition by means of a pump 9. However, after passing through the gap or restricted orifice 5, the coating colour mixture is allowed to collect in the area or nip 7 formed between the blade 3 and partition 4 on the one hand and the moving sheet material 1 on the other hand. The amount of coating colour mixture 6 introduced by the pump between blade 3 and partition 4 is regulated in accordance with the amount of coating colour mixture present in nip 7 as determined by level sensing means 8.

Referring to FIGURE 3, there is shown a third embodiment of the invention, like parts of this embodiment being designated by the same reference numerals used in FIGURE 1. In this embodiment, the sheet material 1 moves past the restricted orifice 5 in an upward direction, the coating colour mixture or composition 6 being pumped through the gap 5 defined by blade 3 and partition 4 and into contact with the sheet material 1. Any excess coating colour mixture passing through the gap 5 runs over the gap-defining edge of partition 4 and falls between the sheet material 1 and the partition 4. This excess can be collected and recirculated.

What we claim is: 1. A method of applying a coating composition to a sheet material comprising the steps of pumping a coating composition to a receiving area;

metering said coating composition from said receiving area through a restricted orifice to an application area between said receiving area and said sheet material and contiguous with moving sheet material, the receiving area and application area being contiguous and separated by a partition having a restricted orifice so as to limit the amount of coating composition in contact with the moving sheet material;

measuring the level of said coating composition in said application area to determine the amount of said coating composition in said application area;

regulating the amount of said coating composition introduced into said receiving area and thereby regulating the amount of said coating composition metered into said application area;

moving said sheet material in a predetermined direction With said application area;

coating said moving sheet material with said coating composition; and

scraping said coating composition with a blade in order to obtain the desired thickness of said composition.

2. The method of claim 1, wherein said sheet material is moved in a downward direction.

3. Apparatus for applying a coating composition to a sheet material comprising:

roller means to support and move said sheet material;

scraping blade means operatively associated therewith at an acute angle thereto and in trailing position with relation to the intended direction of movement of said roller means, said scraping blade means having a forward edge adapted to contact the sheet material at a point where said sheet material is supported by said roller means;

a partition angularly inclined with respect to said scraping blade and defining a receiving area between said scraping blade and said partition, said partition being disposed from said roller means so as to define an application area between said roller means and said partition, and said partition having a forward edge disposed adjacent to said scraping blade, so that a restricted orifice is defined therebetween through which a metered amount of said coating composition may be introduced to and deposited on a limited area of said sheet material; pump means operatively associated with said receiving area for pumping said coating composition into said receiving area from which said coating composition is introduced to said application area; and level sensing means disposed between said sheet material and said partition in said application area for measuring the level of said coating composition collected between said sheet material and said partition, and operatively connected to said] pump means for regulating the amount of said coating composition delivered by said pump means to said receiving area. 4. The apparatus of claim 3, in which said roller means is covered by a resilient material.

5. The apparatus of claim 4, in which said resilient material is rubber.

6. The apparatus of claim 3, wherein said partition is 25 flexible.

7. The apparatus of claim 6 in Which said flexible partition comprises a diaphragm.

References Cited UNITED STATES PATENTS 2,251,295 8/1941 Sheesley. 2,887,087 5/1959 Jones l187 2,406,470 8/1946 Maxim et al 118-7 3,070,066 12/1962 Faeber 118-413 3,179,536 4/1965 Martinek l18126X 3,273,535 9/1966 Krikorian 118413 X ALFRED L. LEAVITT, Primary Examiner C. R. WILSON, Assistant Examiner U.S. Cl. X.R. 1187, 413, 419 

